This invention relates to chemically etching steel and in particular to a method and composition for chemical etching and milling effective on a wide variety of steel alloys.
Applicants are aware of the following U.S. Pat. Nos. 2,177,751, 2,434,021, 2,647,864, 2,650,157, 3,033,795, 3,171,767, 3,230,172, 3,301,719, 3,367,874, 3,479,293, 4,264,377, 4,284,468.
The disclosures of the above patents are incorporated by reference herein.
There are numerous methods and compositions known for etching steels. These prior methods and compositions generally are limited in effectiveness to a particular steel or type of steel and are ineffective on a variety of steels. Thus it has been necessary to select an etchant for the particular steel to be etched. This often requires the shipping, storing, and handling of a large number of dangerous etchants.
Even where the proper prior etchant was selected, etching was slow and difficult to control. It has been particularly difficult to achieve a uniform etch with the prior etchants, especially with the faster acting etchants. Thus, it has been difficult to chemically mill steel parts, especially for high precision and high volume applications.
Generally, the prior etchants have been based upon nitric acid, and their use has resulted in the liberation of large volumes of oxides of nitrogen. The prior etchants have also typically contained chromates or other potential pollution sources that have made their use dangerous and their disposal difficult.
The present invention is a method and composition for etching steel. Applicants' etchant has been found effective on a wide range of steels, including all but the high alloy nickel-chromium stainless steels. The composition has been found effective for chemically etching and milling a wide variety of steels, including 4130, 4140, 4340, 4330, 300M, H-HP9-4-30, D6AC, 1018, and other low and intermediate steel alloys. Thus the composition eliminates the problems of shipping, storing, and handling a wide variety of etchants to work with multiple types of steels. The composition is very fast compared to prior etchants. The etching is very uniform and leaves a very smooth etched surface, including at fillets and similar structures. Thus the etchant is particularly suited for chemical milling of steels and parts fabricated of steel.
The composition includes nitric acid and hydrofluoric acid. The new etchant incorporates a variety of etch control additives, such as urea, to keep metal surfaces and fillets smooth by buffering the acidic reaction. Urea also inhibits the formation of nitrogen oxides usually generated in the chemical milling of steel with nitric acid. Sodium benzoate, in particular, is added to achieve a more uniform etch rate. Phosphoric acid may also be added to achieve a smooth surface finish. Sodium nitrate can be added to supply additional sodium ions to retard the formation of scale on the steel part being treated and on the treatment tank. Iron can be added to the composition to help achieve a uniform etch rate, so either a small amount of iron or steel can be dissolved in the composition. Thus the composition can be used in precision chemical etching and milling. The more uniform etch permits stronger, faster etching compositions to be used, reducing treatment times.
The method of chemical etching or milling of this invention includes the preparation of a sufficient quantity of the composition in a suitable container or tank. The steel parts to be etched or milled are immersed in the tank for the appropriate period of time to remove the desired amount of material. The components of the etchant, such as nitric acid and hydrofluoric acid, can be replenished as they are consumed during etching or milling of the steel. In a preferred composition, and method of using the composition, a liter of the etchant contains between about:
______________________________________ 10-300 ml 75% H.sub.3 PO.sub.4 100-700 ml HNO.sub.3 (42.degree./Be) 80-100 ml HF (70%), 1-50 g NaNO.sub.3, 10-100 g Urea, 1-30 g Na Benzoate 5-200 g iron or steel balance water ______________________________________
A preferred composition, which has been found to be particularly effective, contains on a per liter basis:
______________________________________ 100 ml 75% H.sub.3 PO.sub.4 500 ml HNO.sub.3 (42.degree./Be) 100 ml HF (70%) 40 g Urea 10 g NaNO.sub.3 4 g Na Benzoate 40 g dissolved steel Balance water ______________________________________